The present invention involves a method of priming SMC parts. More particularly, the present invention comprises compositions and methods of priming SMC parts with a light curable, conductive priming composition.
Sheet molding compound (SMC) is a moldable composite material generally made up of an unsaturated liquid polyester resin, a low profile thermoplastic resin, an inert filler, a curing aid, and short lengths of glass fiber reinforcing materials. The constituents are mixed on a film of polyethylene or other material. The mixture is covered with a second film. The SMC sandwich (film-SMC-film) is allowed to age until the polyester resin matures to a putty-like consistency.
Prior to molding, the protective sheets are removed and the matured SMC is cut into pieces of appropriate size for a particular mold. The SMC pieces are laid on a bottom mold of a matched set of top and bottom compression molds in a compression press. Heated molds are closed to mold a part from the piece of SMC.
SMC parts are used in many types of products including vehicles, appliances, business equipment, and recreational equipment. In automotive applications, SMC parts are frequently used to replace sheet metal parts, such as fenders, doors, hoods, and roofs. Some of the advantages of SMC parts over metal parts are that SMC resists corrosion and deterioration from weathering that presents significant problems with the use of metals. SMC parts are more resistant to denting than metals. Furthermore, SMC parts can be economically molded into intricate parts that can not be done with metals.
Conley et al., U.S. Pat. No. 3,679,510 shows a lamination of a relatively thick thermoplastic sheet with a polyvinyl fluoride outer surface to an SMC sheet to form a laminate. Such laminates are not useful for many automotive and truck applications that require high modulus materials for rigidity and thermal stability such as automobile and truck hoods. Attempts to reduce the thickness of the thermoplastic layer with the methods taught by Conley et al. have resulted in imperfections caused by the glass fibers in the SMC layer extending through the surface of the parts. As such, parts formed using such a procedure are not acceptable to produce a class A surface for automotive or truck use.
After molding SMC into a desired shape, the surface of the SMC part may contain pits, porosity and/or microcracks. As such, SMC parts generally do have a smooth, shiny class A surface that can be readily finished or painted. However, pits, porosity and microcracks can require more than one primer coat application to obtain a defect free surface. To obtain a smooth, paintable surface of high quality, the surface of the SMC part is usually cleaned, primed, and then painted with one or more additional coats. These additional coats are often applied in an electrostatic spraying operation that needs the part being painted to be conductive. Examples of additional, finish, coats that can be applied after priming include primer/sealer coats, base color coats, and clear coats. Even with multiple primer coats using current conventional priming technology, unacceptable defects can appear later during the manufacturing process.
The painting methods used in the above application need to have good transfer efficiencies in order to provide coated (or painted) parts in a cost-effective way. Further, SMC parts, which are made of thermoset material, pose particular painting problems because materials and methods that work well with other body part material such as metal (including, but not limited to steel and aluminum) or thermoplastic materials do not necessarily work well with SMC parts. On the other hand, techniques that work with SMC parts generally work with metal parts, including steel parts in particular.
SMC parts are currently primed with solvent-based primers. Once the primers are applied to the SMC parts, the primed parts are placed in an oven. Heating cures the primer by causing the solvent to be evolved from the primed SMC parts. One problem associated with this process is that the solvent must be recovered rather than emitted to the atmosphere because of environmental concerns.
Additionally, problems develop when a portion of the solvent remains trapped in thepits, porosity, and microcracks. When the SMC parts are used, such as in an automotive body panel, heat causes any solvent that remains in the primer to be evolved. Evolution of the solvent creates small cracks in finish layers on the SMC parts. While these cracks are typically too small to see without magnification, the cracks eventually degrade the appearance and useful life of the SMC parts. If more than minimal quantities of solvent are trapped on the primed part, unacceptable xe2x80x9cpaint popsxe2x80x9d can be created in the finished product, ruining the appearance of the part immediately. Such paint-related defects in final painted products commonly occur with established SMC part priming techniques.
Sokol, U.S. Pat. No. 5,773,487 describes a solvent-free, coating composition that is curable with ultraviolet light. The Sokol coating composition contains an acrylate mixture and a photoinitiator. The Sokol patent, however, does not disclose a conductive component that would make the invention of the Sokol patent useful in electrostatic spraying operations.
It would be beneficial to have compositions and methods to that address the needs still unfulfilled by the present methods for painting that use primers that, do not evolve volatile organic compounds that can harm the environment or require expensive manufacturing controls, have good transfer efficiencies, reliably produce low defect Class A surfaces, and are usable with good transfer efficiencies in electrostatic spraying operations. The present invention addresses some of these needs highlighted by the limitations of current methods, and offers further benefits that are described herein.